Differential Equations
Curve through point — slope field
Grade Class 12
Question:
<p>Curve through \\((1,-2)\\), slope \\(=\\dfrac{x^2-2y}{x}\\). Find \\(y\\) at \\(x=-1\\).</p>
<span>\(-3\)</span>
<span>\(3\)</span>
<span>\(-\frac{5}{2}\)</span>
<span>\(\frac{5}{2}\)</span>
Step-by-Step Solution
Key Concept: Linear ODE: dy/dx + 2y/x = x. Find particular solution.
Step 1:
The slope of the curve is given by $\frac{dy}{dx} = \frac{x^2 - 2y}{x}$.
This can be rewritten as a linear first-order differential equation:
$$ \frac{dy}{dx} = x - \frac{2y}{x} $$
$$ \frac{dy}{dx} + \frac{2}{x}y = x $$
This is in the form $\frac{dy}{dx} + P(x)y = Q(x)$, where $P(x) = \frac{2}{x}$ and $Q(x) = x$.
The integrating factor (IF) is $e^{\int P(x) dx} = e^{\int \frac{2}{x} dx} = e^{2 \ln|x|} = e^{\ln(x^2)} = x^2$.
Multiplying the differential equation by the integrating factor:
$$ x^2 \frac{dy}{dx} + 2xy = x^3 $$
The left side is the derivative of the product $x^2y$:
$$ \frac{d}{dx}(x^2y) = x^3 $$
Integrating both sides with respect to $x$:
$$ \int \frac{d}{dx}(x^2y) dx = \int x^3 dx $$
$$ x^2y = \frac{x^4}{4} + C $$
Step 2:
The curve passes through the point $(1, -2)$. Substitute $x=1$ and $y=-2$ into the general solution to find the constant $C$:
$$ (1)^2(-2) = \frac{(1)^4}{4} + C $$
$$ -2 = \frac{1}{4} + C $$
$$ C = -2 - \frac{1}{4} = -\frac{8}{4} - \frac{1}{4} = -\frac{9}{4} $$
Thus, the particular solution is:
$$ x^2y = \frac{x^4}{4} - \frac{9}{4} $$
Solving for $y$:
$$ y = \frac{x^4}{4x^2} - \frac{9}{4x^2} $$
$$ y = \frac{x^2}{4} - \frac{9}{4x^2} $$
Step 3:
To find the ordinate of the point on the curve with abscissa $x = -1$, substitute $x=-1$ into the particular solution:
$$ y(-1) = \frac{(-1)^2}{4} - \frac{9}{4(-1)^2} $$
$$ y(-1) = \frac{1}{4} - \frac{9}{4(1)} $$
$$ y(-1) = \frac{1}{4} - \frac{9}{4} $$
$$ y(-1) = -\frac{8}{4} $$
$$ y(-1) = -2 $$
Correct Answer: 1